2 Answers
2

Hindering terrain is defined in the new Rules Compendium (207). It is terrain that punishes creatures for entering/being in them.

The examples of hindering terrain are pits, electrifying runes, lava and deep water.

A creature can make a saving throw when it is pulled, pushed, slid, or teleported into hindering terrain.

In short, hindering terrain is whatever the DM or module says is hindering terrain. When a DM describes the battlefield to the players he should be explicit as to which terrain that they can see is hindering.

Zones are not hindering terrain.

Cliffs are not
hindering terrain. Forced movement
grants a saving throw if the target
is forced over a precipice or into hindering terrain (Rules Compendium p212).

An earlier definition of Hindering Terrain is in the Dungeon Masters Guide p61. That definition says that Hindering Terrain prevents movement or severely punishes it. Preventing movement is no longer part of the definition of Hindering Terrain. Preventing movement is handled under Blocking Terrain.

You can probably add gimmicky teleports to the list of pseudo-hindering terrain. I teleport you 6 squares straight up? Lets roll a save first.

Anything that either does massive damage (ie more damage than a daily, ie lava, drowning, etc) or instantly removes/reduces a creatures ability to fight by a massive margin (consider a pit more as soemthing that would require a minute and maybe some help to get out of, removing a creature for a minute from combat is amazing for a power rather than a pit trap).